cmake: set.cmake: fix qi_persistent_{pre,ap}pend_uniq functions

Because qi_persistent_prepend_uniq and qi_perssistent_append_uniq are
functions, they must store their result in their parent scope;
otherwise, the result is lost as soon as the function is exited.

Change-Id: I6fd107b4d8ba7e6edec931e31f28218839ab2c1a
Reviewed-on: http://gerrit.aldebaran.lan:8080/9748
Reviewed-by: cgestes <gestes@aldebaran-robotics.com>
Reviewed-by: smartin <smartin@aldebaran-robotics.com>
Tested-by: smartin <smartin@aldebaran-robotics.com>
cmake: set.cmake: cleanup deprecation messages

Do not recommend usage of non-existing functions or macros in the
deprecation messages.

Change-Id: I94987710a430ce9914025578be6568c35085f988
Reviewed-on: http://gerrit.aldebaran.lan:8080/9749
Reviewed-by: cgestes <gestes@aldebaran-robotics.com>
Reviewed-by: smartin <smartin@aldebaran-robotics.com>
Tested-by: smartin <smartin@aldebaran-robotics.com>